Permanent magnet system for producing a magnetic field for the focused passage of a beam of electrons



Feb. 22, 1966 P. MEYERER 3,237,059

PERMANENT MAGNET SYSTEM FOR PRODUCING A MAGNETIC FIELD FOR THE FOCUSEDPASSAGE OF A BEAM OF ELECTRONS Filed Sept. 23, 1963 3 Sheets-Sheet lFeb. 22, 1966 MEYERER 3,237,059

PERMANENT MAGNET SYSTEM FOR PRODUCING A MAGNETIC FIELD FOR THE FOCUSEDPASSAGE OF A BEAM OF ELECTRONS Filed Sept. 25, 1965 3 Sheets-Sheet 2Fig.3

Feb. 22, 1966 MEYERER 3,237,059

PERMANENT MAGNET SYSTEM FOR PRODUCING A MAGNETIC FIELD FOR THE FOCUSEDPASSAGE OF A BEAM 0F ELECTRONS Filed Sept. 23, 1963 3 Sheets-Sheet 5United States Patent 3 237,059 PERMANENT MAGNE T SYSTEM FOR PRODUCHNG AMAGNETIC FIELD FOR THE FOCUSED PAS- SAGE OF A BEAM OF ELECTRONS PaulMeyerer, Ottobrunn, near Munich, Germany, assignor to Siemens & HalskeAktiengescllschaft, Berlin and Munich, a corporation of Germany FiledSept. 23, 1963, Ser. No. 310,802 Claims priority, application Germany,Oct. 4, 1962,

9 Claims. (Cl. s17 20e The present invention relates vto a system ofpermanent magnets for producing a substantially homogeneous magneticfield for the focused passage of a beam of electrons over an extendedpath, especially for high power travelingwave tubes, comprisingpermanent magnets which are arranged with axial symmetry about the axisof the system, and are magnetized radially to the axis of the system,and the outwardly disposed poles of which are magneticallyshort-circuited in the longitudinal direction of the system, there beingprovided between the magnet poles of opposite polarity arranged at thetwo ends of the system in the longitudinal direction thereof, furthermagnet poles which are respectively of the same polarity up to thecenter of the system, which magnet poles efiect a homogenizing of thecourse of the magnetic field, at least in the region of said magnetpoles.

In electronic tubes for very high frequencies, and particularly intraveling Wave tubes, it is, as is known, fre quently necessary toconduct a beam of electrons over an extended path and focused in such amanner that the diameter of the beam of electrons remains as constant aspossible. it is known to use magnetic coils for this purpose. The fieldstrength on the axis of a coil magnet is not homogeneous, but greater inits center that at its ends. One known method of homogenizing the coilfield consists in arranging the winding of the magnet coil on asupporting body of step-shaped profile, the number of ampere conductorsper inch increasing away from the center of the coil.

' Magnetic coils have the disadvantage that they consume electric power.The tendency therefore has been increasingly toward using permanentmagnet systems for the focused guiding of the beam of electrons of avelocitymodulated tube. Tubular and barrel-magnets are known for use aspermanent magnets for the production of a substantially homogeneousmagnetic field for the focused guiding of a beam of electrons. In order,in the case of a system of permanent magnets having a tubular,cylindrical permanent magnet, to homogenize the magnetic field on theaxis of the system, it is known to provide within the magnet a hollowcylinder of ferromagnetic material, the cross-section of which tapersfrom the center at least toward one end. It is likewise known, inconnection with a system of tubular permanent magnets, in order toobtain a homogeneous distribution of the magnetic field, to compose thetubular magnet of two oppositely, radially magnetized tube parts whichin part demagnetize each other, thus bringing about the desired magneticpotential distribution along the axis of the magnetic system.

In all of these known systems employing a tubular or barrel magnets,there is a considerable magnetic scattering flux or leakage in the outerspace of the magnet so that very heavy magnets of high energy contentare necessary, the magnetic induction of which contributes only to aslight extent to the useful magnetic flux.

In order to suppress the scattering flux in the outer space of apermanent-magnet focusing system, it is known to develop the focusingsystem of radially magnetized permanent magnets which are respectivelygrouped at the beginning and end of the system in star shape around the3,237,059 Patented Feb. 22, 1966 axis of the system and rest with theirouter ends against pieces of soft iron which extend in axial directionand magnetically connect together the outer poles of the permanentmagnets. In order to obtain thereby sufiicient homogeneity of themagnetic field on the axis of the sys tem, the inner ends of .thepermanent magnets are connected by a tubular shell of soft iron. As aresult of the presence of the tubular shell of soft iron, a considerablepart of the magnetic induction again does not contribute toward thefocusing of the beam of electrons.

In order to avoid the tubular shell of soft iron, it is finally known,in one such system of permanent magnets, to provide, at a slightdistance from the permanent magnets which are arranged at the two endsof the system, and parallel with them, another set of magnet barsarranged in star-shape with respect to the axis of the system, and whichare magnetized in the same direction as the permanent magnets arrangedat the ends of the system. These additional magnet bars should be sodimensioned that the focusing magnetic field is homogenized in theirvicinity. A particularly good homogenization of the course of themagnetic field over the entire length of the magnet system, such as isobtainable by means of a soft-magnetic guide tube, is, however, notobtained hereby. The course of the magnetic field, rather, still showsnoticeable undulations so that the system of permanent magnets is notsuitable for the focused guiding of a beam of electrons over an extendedpath (for instance more than 20 cm). Furthermore, the correspondingconstruction has a considerable internal scatting fiux which necessarilydraws energy from the two sets of permanent magnets arranged at the endsof the system. For thesepermanent magnets, there are therefore againrequired magnet materials which have a considerable magnetic energycontent.

The object of the present invention is to create a system of permanentmagnets for obtaining a substantially homogeneous magnetic field for thefocused passage of a-beam of electrons over an extended path, andparticularly for highpower traveling wave tubes, in which the Weight ofthe magnet, for a given useful flux, can be substantially reduced ascompared with the known permanent magnets. To achieve this purpose,there is proposed a permanent-magnet system of the initially mentionedkind in which, in accordance with the invention, the permanent magnetsform on their side facing the system axis, a plurality of magnetic polesfollowing closely upon each other, the magnetic potential of whichdecreases in such a manner from the ends toward the center of the systemthat all lines of force extending from the individual magnetic polesextend inside the system substantially parallel to the system axis.

The invention makes it possible to use for the permanent magnets amaterial which has a low energy content and a very high coercive force(I-I 2000 oersteds). The reason for this is that in the case of apermanent-magnet system in accordance with the invention, the ratio ofthe magnetic useful flux to the magnetic scattering flux is very highand therefore essentially only the energy requirement of the magneticuseful flux need be covered. By material of high coercive force and lowenergy content, there is to be understood a magnet material in which thequotient of the magnetic induction at the operating point B A divided bythe magnetic field strength at the operating point H is of the order ofmagnitude of a one place number. As magnet materials of this type,hardmagnetic ferrites or alloys having a platinum-cobalt base are, forinstance, known. In accordance with the known laws of the dimensioningof permanent magnets of a material of high coercive force and low energycontent, it follows that such permanent magnets are compact, thusresulting in a low magnet volume. Moreover, when using hard-magneticferrites instead of the Alnico alloys heretofore customary forpermanent-magnetic focusing systems, the weight of a permanent magnet inaccordance with the invention becomes even less since ferrites have alow specific weight.

A permanent magnet system in accordance with the inventionadvantageously consists of permanent magnets which are arranged in closesuccession, one behind the other, and the lengths of which continuouslydecrease from the two outer ends toward the center of the system. Theindividual permanent magnets are, in this connection, as close aspossible to each other. In the border case, there is obtained a unitarypermanent magnet which tapers from the end of the system to the outsideof the system in a step-wise, or wedge-shape manner; The permanentmagnets which are magnetized radially to the axis of the system may beannular or grouped as magnet bars in star-shape around the axis of thesystem.

Further details of the invention will now be explained with reference tothe accompanying drawings showing embodiments thereof.

FIG. 1 shows a longitudinal section through a permanent magnet systemaccording to the invention;

FIG. 2 indicates the course of the field of a permanent magnet systemaccording to FIG. 1;

FIG. 3 represents an end view of a permanent magnet system according tothe invention;

FIG. 4 is a longitudinal section through the system represented in FIG.3; and

FIG. 5 shows a permanent magnet system comprising reinforcing endmagnets for supplying the energy requirements caused by unavoidablescattering flux.

Referring now to FIG. 1, in a cylindrical soft-iron tube 1, there arearranged radially magnetized rings 2 which consist for instance ofhard-magnetic ferrite of high coercive force. The inner diameter of therings 2, the magnetic poles of which, facing the axis of the system, arein each case of the same polarity as each other from the end of thesystem to the center, increases progressively from the two ends of thesystem toward the center thereof. The height of the individual rings 2is so dimensioned that their magnetic poles, facing the axis of thesystem, have a magnetic voltage of such a value that the magnetic linesof potential in the entire inner space of the system of permanentmagnets are substantially perpendicular to the axis of the system andhave practically the same spacing from each other. This potentialpattern is indicated by the lines 3, the numerals which are provided ineach case with a positive or negative sign in the magnet poles of therings 2 facing the magnet axis, representing normalized values of themagnetic voltage. The result is then that all lines of force extendingfrom the individual magnetic poles extend within the systemsubstantially parallel to the axis of the system since, as is known,magnetic lines of force always are at right angles to the lines ofpotential. The lines 4 indicate how the magnetic lines of flux extendbetween the two magnetic poles arranged at the ends of the magnetsystem. The field distribution is in this connection more uniform, thefiner the grading of the permanent magnets is effected. In the outerspace of the system of magnets, there is no scattering flux at all,along the magnet system (in contradistinction to a known tubular orbarrel magnet), due to the magnetic short-circuit represented by thesoft-iron cylinder 1. Only at the two ends of the system is there aslight scattering flux which is indicated by the field lines 5. Theresult thus is that for a given required induction along the axis of thesystem of magnets (for instance an induction of 500 G), the energycontent of the permanent magnets can be substantially less even over anextended path (or instance greater than 20 cm.) than in the case of theknown initially described permanent magnet systems.

Below FIG. 1, there is shown in FIG. 2 the course of the field of asystem of permanent magnets in accordance with FIG. 1, the x-axis of therectangular coordinate system indicating the path 2 in axial directionof the system and the y-axis indicating the magnetic induction B Thefull line curve 6 then represents the value of the magnetic field 13along the axis of the focusing system of FIG. 1. The dashed curve 7indicates that in the case of a permanent-magnet system in accordancewith FIG. 1, the field can be increased by a corresponding increase inthe magnetic voltage on the output side of the magnet system, asrequired at times for traveling-wave tubes of high power.

In the permanent magnet system shown in FIGS. 3 and 4, there are used,instead of the annular permanent magnets 2 of FIG. 1, magnet bars 8which are arranged in star-shape around the axis of the system and havetheir outer poles resting against soft iron bridges 9. In a mannersimilar to FIG. 1, the length of the individual magnet bars which arearranged in a row in close succession in the longitudinal direction ofthe system, decreases from the two ends toward the center of the system.With suitable dimensioning of the length of the magnet bars 8, there isagain obtained a potential distribution such as that shown in FIG. 1.The individual soft iron rails 9 are connected with one another by asoft iron sheet 10 to form a screening housing which surrounds thesystem of magnets. The occurrence of scat tering flux in the outer spaceof the system, aside from the scattering flux at the ends, is in thisway prevented. Numerals 11 indicate pole shoes around which are arrangedthe magnet bars on the two ends of the system and into which a travelingwave tube can be inserted. It may be pointed out that such pole shoescan also be used in permanent magnet systems, such as shown in FIG. 1.

It is in connection with a permanent magnet system according to thepresent invention, particularly advantageous to cover the energyrequirement of the fundamentally unavoidable small scattering flux attwo ends of the permanent magnet system, by reinforcing the twopermanent magnets, arranged at the ends of the system by means ofadditional magnets which are also radially magnetized. FIG. 5 shows anembodiment utilizing this feature.

The permanent magnet system of FIG. 5 is substantally similar to thesystem shown in FIGS. 3 and 4. The two magnetic bars 12 disposed at theends are merely reinforced by further magnetic bars 13. FIG. 5 showsboth, the case in which the magnetic bars 13 have the same as well asthe case in which they have a smaller magnetic voltage than the adjacentmagnetic bars 12, depending on how high the energy requirement to coverthe outer scattering flux is.

The invention is not inherently limited to the illustrated embodiments.In particular, it is not necessary for the permanent magnets to bestepped-down on the inside, as it is possible that they also be steppeddown on the outside of the system. In such a case, the soft-magenticscreening covering must have a shape corresponding to the mode ofstepping. Furthermore, more than four magnet bars can be grouped in aplane at right angles to the axis of the system in star-shape about suchaxis. It is likewise possible to use instead of hard-magnetic ferrites,any other suitable and known material of high coercive force and lowenergy content.

Changes may be made within the scope and spirit of the appended claimswhich define what is believed to be new and desired to have protected byLetters Patent.

I claim:

1. A permanent-magnet system for producing substantially homogeneousmagnetic field for the focused passage of a beam of electrons over anextended path, particularly for high power traveling-wave tubes,comprising permanent mangents disposed along and arranged with axialsymmetry about the axis of the system, said magnets being magnetizedradially to the system axis, means at the outer poles of said permanentmagnets for magnetically short circuiting such poles in the longitudinaldirection of the system, said permanent magnets being so disposed thatthe poles disposed at one end of the system have a polarity which isopposite to that of the corresponding poles at the other end of thesystem, said permanent magnets forming further magnetic poles arrangedin the longitudinal direction of the systern, between said magneticpoles of opposite polarity arranged at the two ends of the system, therespective further magnetic poles being of the same polarity up to thecenter of the system and effecting a homogenizing of the course of themagnetic field, at least in the vicinity of said magnet poles, saidpermanent magnets forming on the side thereof which faces the axis ofthe system, a plurality of magnetic poles closely following one another,the magnetic potential of which decreases from the ends toward thecenter of the system so that all lines of flux extending from individualmagnetic poles pass within the system substantially parallel to thesystem axis.

2. A permanent magnet system according to claim 1, wherein the permanentmagnets are made of hard-magnetic ferrite material.

3. A permanent magnet system according to claim 2, wherein the permanentmagnets have, as seen from the two ends toward the center of the system,a progressively decreasing height.

4. A permanent magnet system according to claim 3, wherein the permanentmagnets are stepped down stepwise, the outer ends of the permanentmagnet lying in a common plane.

5. A permanent magnet system according to claim 4, wherein theindividual steps are respectively formed by annular radially magnetizedpermanent magnets which are arranged in a row in the longitudinaldirection of the system.

6. A permanent magnet system according to claim 4, wherein theindividual steps are respectively formed by magnet bars arranged instar-shape about the axis of the system, said bars being located onebehind the other in the longitudinal direction of the system.

7. A permanent magnet system according to claim 5, wherein the permanentmagnets, which are arranged at the two ends of the system, arereinforced by additional magnets which are likewise radially magnetizedand which supply the energy for the external scattering flux.

8. A permanent magnet system according to :claim 7, wherein theadditional magnets have magnetic potentials which are not greater thanthose of the adjacent permanent magnets.

9. A permanent magnet system according to claim 1, wherein the magneticshort-circuiting of the outer poles of the permanent magnets is effectedvia elongated softiron rails which are connected together to form ahousing surrounding the system.

References Cited by the Examiner UNITED STATES PATENTS 2,876,373 3/1959Vieth et al. 313-84 FOREIGN PATENTS 770,133 3/1957 Great Britain.

OTHER REFERENCES Parker et al.: Permanent Magnets and Their Application,New York, John Wiley and Son, Inc., 1962, QC. 757 F 37, pp. 271-274.

ROBERT K. SCHAEFER, Acting Primary Examiner.

JOHN F. BURNS, Examiner.

1. A PERMANENT-MAGNET SYSTEM FOR PRODUCING SUBSTANTIALLY HOMOGENEOUSMAGNETIC FIELD FOR THE FOCUSED PASSAGE OF A BEAM OF ELECTRONS OVER ANEXTENDED PATH, PARTICULARLY FOR HIGH POWER TRAVELING-WAVE TUBES,COMPRISING PERMANENT MANGENTS DISPOSED ALONG AND ARRANGED WITH AXIALSYMMETRY ABOUT THE AXIS OF THE SYSTEM, SAID MAGNETS BEING MAGNETIZEDRADIALLY TO THE SYSTEM AXIS, MEANS AT THE OUTER POLES OF SAID PERMANENTMAGNETS FOR MAGNETICALLY SHORT CIRCUITING SUCH POLES IN THE LONGITUDINALDIRECTION OF THE SYSTEM, SAID PERMANENT MAGNETS BEING SO DISPOSED THATTHE POLES DISPOSED AT ONE END OF THE SYSTEM HAVE A POLARITY WHICH ISOPPOSITE TO THAT OF THE CORRESPONDING POLES AT THE OTHER END OF THESYSTEM, SAID PERMANENT MAGNETS FORMING FURTHER MAGNETIC POLES ARRANGEDIN THE LONGITUDINAL DIRECTION OF THE SYSTEM, BETWEEN SAID MAGNETIC POLESOF OPPOSITE POLARITY ARRANGED AT THE TWO ENDS OF THE SYSTEM, THERESPECTIVE FUTHER MAGNETIC POLES BEING OF THE SAME POLARITY UP TO